In the novel's beginning Henry is just a naive, young man. War is romantic- a great adventure. He believes in courage and honor, and he seeks glory; not in the sense of fighting for what's right but rather fighting to gain a reputation. It's all about the glory.
When Henry is tested in his first battle, things begin to change a bit. He flees the scene and then justifies his own actions, all while demeaning the men who were courageous enough to stay by deeming them imbeciles, "not wise enough to save themselves from the flurry of death.”
Henry's next foray into battle is different; instead of running away from the actions.......... he charges directly into it. He becomes a machine, marked by desire to survive and a great sense of urgency. It was during this battle that he becomes the hero he so wanted to be, but he does it for the right reasons, and in the end he's earned the right to be called such.
Henry's final transformation into manhood comes with his ability to admit his mistakes and see the things he would have done differently. Thus, he becomes a man when he can finally admit that he was a naive and unknowing youth.